This is a good question that seems to pop up quite often. Of course, up to CCM 5.x licensing is really honor based so there is no report that you can query that will give you this info. Have a look at the following threads that go over this in some detail. Sankar really explains this very well!

Q. Do I need a license for a Cisco IP Phone, and how does the licensing scheme work?

A. Customers of the Cisco CallManager software are required to purchase a CallManager Client Access License (CAL) for each IP phone, SoftPhone, or other endpoint device or application (IP phone) that registers with the CallManager software. This CAL requisite applies regardless of the source (Cisco or a third party) or the functionality of that IP phone.

Customers who purchase a spare IP phone need not purchase a CAL for that spare if that spare is used to replace another IP phone for which a CAL has already been paid.

Note: That spare phone is never registered with a Cisco CallManager until the phone that the spare replaces has been deregistered with that CallManager.

If the spare IP phone is not used as a replacement for an IP phone for which a CAL has already been paid, the customer must purchase a CAL for that spare. The Cisco CallManager CAL policy is that customers must purchase a matching number of IP phones and associated CALs, with the exception of spares that are used as replacements.